The present invention relates to a semiconductor device having an insulating gate using a nitride semiconductor for an active layer thereof and to a method for fabricating the same.
FIG. 19 shows a cross-sectional structure of a conventional Schottky-gate field effect transistor (FET) which is composed of Group III-V nitride semiconductors.
As shown in FIG. 19, a channel layer 102 composed of gallium nitride (GaN) and a carrier supply layer 103 composed of n-type aluminum gallium nitride (AlGaN) are formed successively on a substrate 101 composed of sapphire. A two-dimensional electron gas layer composed of a potential well and having an extremely high electron mobility is formed adjacent the heterojunction between the upper portion of the channel layer 102 and the carrier supply layer 103. For this reason, the FET is also termed a high electron mobility transistor (HEMT).
In the foregoing conventional Schottky-gate FET, however, the breakdown voltage of the gate electrode is determined by the Schottky characteristic thereof so that the reverse breakdown voltage of the gate electrode is limited. In addition, a forward voltage applied to the gate electrode is also limited to about only 2 V so that a high-output semiconductor device (power device) having a high current driving ability is not obtainable.
It is therefore an object of the present invention to enhance the current driving ability of a semiconductor device having a gate electrode and composed of nitride semiconductors by solving the foregoing conventional problems.
To attain the object, the present invention uses an insulating gate as a gate electrode in the semiconductor device composed of nitride semiconductors and forms a gate insulating film by oxidizing a deposited nitride semiconductor.
Specifically, a semiconductor device according to the present invention comprises: a first nitride semiconductor layer formed over a substrate; an insulating oxidation layer obtained by oxidizing a second nitride semiconductor layer formed on the first nitride semiconductor layer; and a gate electrode formed on the insulating oxidation layer.
In the semiconductor device according to the present invention, the insulating oxidation layer has an excellent film quality and an extremely clean interface in contact with the underlying first nitride semiconductor layer. As a result, a leakage current seldom flows in the gate electrode formed on the insulating oxidation layer and the current-voltage characteristic of the semiconductor device is not limited by the Schottky characteristic of the gate electrode so that a high breakdown voltage and a high current driving ability are achievable.
In the semiconductor device according to the present invention, an oxidation rate for the first nitride semiconductor layer is preferably lower than an oxidation rate for the second nitride semiconductor layer. The arrangement facilitates selective oxidation of only the second nitride semiconductor layer during the fabrication process.
In the semiconductor device according to the present invention, each of the first and second nitride semiconductor layers is preferably composed of the same material.
In the semiconductor device according to the present invention, the first nitride semiconductor layer preferably contains aluminum. Aluminum gallium nitride (AlGaN) obtained by thus doping gallium nitride, which is a typical nitride semiconductor material, with aluminum is resistant to oxidation during the formation of the insulating oxidation layer because of its oxidation rate lower than that of gallium nitride. In addition, aluminum gallium nitride forms a potential barrier layer because of its energy gap larger than that of gallium nitride.
Preferably, the semiconductor device according to the present invention further comprises an active layer formed between the substrate and the first nitride semiconductor layer and composed of a third nitride semiconductor having an energy gap smaller than in the first nitride semiconductor layer. This surely implements a high electron mobility transistor (HEMT) having a high breakdown voltage and a high current driving ability in which the first nitride semiconductor layer serves as the carrier supply layer and the third nitride semiconductor layer serves as a channel layer.
Preferably, the semiconductor device according to the present invention further comprises an anti-oxidation layer formed between the first nitride semiconductor layer and the insulating oxidation layer and composed of a fourth nitride semiconductor having an oxidation rate lower than an oxidation rate for the second nitride semiconductor layer. In forming the insulating oxidation layer by oxidizing the second nitride semiconductor layer in the arrangement, the oxidation is substantially halted by the fourth nitride semiconductor layer so that the film thickness of the insulating oxidation layer serving as a gate insulating film is controlled easily.
In this case, the anti-oxidation layer is preferably composed of aluminum nitride.
Preferably, the semiconductor device according to the present invention further comprises an insulating film formed between the insulating oxidation layer and the gate electrode. The arrangement positively suppresses a leakage current flowing in the gate electrode, allows a high voltage to be applied to the gate electrode, and thereby further enhances the current driving ability of the semiconductor device.
In this case, the insulating film is preferably composed of a silicon oxide film or a silicon nitride film. The arrangement provides a high insulating property since the insulating film has an extremely dense film texture.
Preferably, the semiconductor device according to the present invention further comprises: source and drain electrodes formed in regions of the first nitride semiconductor layer which are located on both sides of the gate electrode, wherein the insulating oxidation layer has a thicker portion which is larger in thickness than a portion of the insulating oxidation layer underlying the gate electrode and located between the gate electrode and at least one of the source and drain electrodes. The arrangement increases the drain breakdown voltage of the drain electrode and reduces a drain leakage current so that the operating voltage of the semiconductor device is increased and the output thereof is increased easily.
A first method for fabricating a semiconductor device comprises: a first step of forming a first nitride semiconductor layer over a substrate; a second step of forming a second nitride semiconductor layer on the first nitride semiconductor layer and oxidizing the formed second nitride semiconductor layer to form an insulating oxidation layer composed of the second nitride semiconductor layer; a third step of forming a gate electrode on the insulating oxidation layer; and a fourth step of performing selective etching with respect to regions of the insulating oxidation layer which are located on both sides of the gate electrode to form openings in the insulating oxidation layer and forming source and drain electrodes in the formed openings.
The first method for fabricating a semiconductor device forms the insulating oxidation layer composed of the second nitride semiconductor layer by oxidizing the second nitride semiconductor layer and forms the gate electrode on the formed insulating oxidation layer so that the semiconductor device according to the present invention is surely obtained.
In the first method for fabricating a semiconductor device according to the present invention, an oxidation rate for the first nitride semiconductor layer is preferably lower than an oxidation rate for the second nitride semiconductor layer.
In the first method for fabricating a semiconductor device according to the present invention, each of the first and second nitride semiconductor layers is preferably composed of the same material.
Preferably, the first method for fabricating a semiconductor device further comprises, prior to the first step, the step of: forming, on the substrate, an active layer composed of a third nitride semiconductor having an energy gap smaller than in the first nitride semiconductor layer.
Preferably, the first method for fabricating a semiconductor device further comprises, between the first and second steps, the step of: forming, on the first nitride semiconductor layer, an anti-oxidation layer composed of a fourth nitride semiconductor having an oxidation rate lower than an oxidation rate for the second nitride semiconductor layer. Since the anti-oxidation layer having the oxidation rate lower than that of the second nitride semiconductor layer is thus formed between the second nitride semiconductor layer forming the insulating oxidation layer as the gate insulating film and the first nitride semiconductor layer formed thereunder, the fourth nitride semiconductor layer is less likely to be oxidized than the second nitride semiconductor layer so that only the second nitride semiconductor layer is oxidized easily. This effects easy control of the film thickness of the insulating oxidation layer serving as the gate insulating film which greatly affects the operating characteristic of the transistor.
In this case, the anti-oxidation layer preferably contains aluminum.
Preferably, the first method for fabricating a semiconductor device further comprises, between the second and third steps, the step of: forming an insulating film on the insulating oxidation layer, wherein the fourth step includes the step of: forming openings also in regions of the insulating film to be formed with the source and drain electrodes.
In this case, the insulating film is preferably composed of a silicon oxide film or a silicon nitride film.
In the first method for fabricating a semiconductor device, the second step preferably includes the steps of: forming the insulating oxidation layer on at least a region of the second nitride semiconductor layer to be formed with the gate electrode; and selectively oxidizing a region between the gate-electrode formation region and a region to be formed with the drain electrode to form the insulating oxidation layer with a thicker portion which is larger in thickness than the insulating oxidation layer.
A second method for fabricating a semiconductor device according to the present invention comprises: a first step of forming a first nitride semiconductor layer over a substrate; a second step of forming a second nitride semiconductor layer on the first nitride semiconductor layer; a third step of forming an anti-oxidation protective film on regions of the second nitride semiconductor layer to be formed with ohmic electrodes; a fourth step of oxidizing the second nitride semiconductor layer by using the anti-oxidation protective film as a mask to form an insulating oxidation layer in the second nitride semiconductor layer except for the ohmic-electrode formation regions; a fifth step of removing the anti-oxidation protective film and forming the ohmic electrodes on the ohmic-electrode formation regions of the second nitride semiconductor layer; and selectively forming a gate electrode on the insulating oxidation layer.
In accordance with the second method for fabricating a semiconductor device, the regions of the second nitride semiconductor layer to be formed with the ohmic electrodes are not oxidized so that the ohmic electrodes are formed without removing the second nitride semiconductor layer. This obviates the necessity to process the second nitride semiconductor layer.
In the second method for fabricating a semiconductor device, the anti-oxidation protective film is preferably composed of silicon. In the second method for fabricating a semiconductor device, the anti-oxidation protective film is preferably an insulating film.
Preferably, the second method for fabricating a semiconductor device further comprises, between the second and third steps, the steps of: forming, on the second nitride semiconductor layer, a protective film covering a region of the second nitride semiconductor layer to be formed with a device; and oxidizing the first and second nitride semiconductor layers by using the formed protective film as a mask to form an isolation film in a peripheral portion of the device formation region, wherein the third step includes the step of: forming the anti-oxidation protective film from the protective film.
Preferably, the second method for fabricating a semiconductor device further comprises, prior to the first step, the step of: forming, on the substrate, an active layer composed of a third nitride semiconductor having an energy gap smaller than in the first nitride semiconductor layer.
Preferably, the second method for fabricating a semiconductor device further comprises, between the first and second steps, the step of: forming, on the first nitride semiconductor layer, an anti-oxidation layer composed of a fourth nitride semiconductor having an oxidation rate lower than an oxidation rate for the second nitride semiconductor layer.
In this case, the anti-oxidation layer preferably contains aluminum.
In the first or second method for fabricating a semiconductor device, the first nitride semiconductor layer preferably contains aluminum.